I have just replaced the sensors on my black tank with the Horst Miracle Sensors. Prior to the change the tank always showed full regardless of whether we had just dumped. Since the change the tank always shows empty. Ground is good and wires are back in the original position (on the new sensors). What went wrong?

i would recheck and make sure a couple wires weren't switched. maybe the top and common. if that is good, see if they are tight.
that is abt all i can suggest. i don't see how anything else could have happened.

there is another item. where three of the wires attach to a small (postage stamp sized) box before the wire goes to the panel. when one of mine was showing full, someone at the factory pointed to that item. don't see how u could have damaged that unless it was pulled on.

if any part of it is still working. the only problem is the one tank, i would verify everything else to be right before replacing a board. if that board is in the display panel, i would think it would be dead on all tanks. i'm a little hard headed abt looking at something other that what i did to stop something from working.

the board may be ur problem but i would try and prove that before buying another. if all u did was swap probes but it was otherwise functioning, i would tend to look elsewhere first. when i couldn't find anything, i would take a bowl of water and place the common lead and then each of the other wires (individually) in it to see it anything changes.

make sure u didn't break one of the wires in the process of tightening them down.

I'd like to try this mod but my trailer has an enclosed underbelly. I'm kind of reluctant to try taking it down. Has anyone done this? If so, I'd like to hear of your experience with it.

We have been using PurePower enzyme additive in the black tank. This year I've been trying the GEO method and driving with just a few gallons of water, softener, and detergent in there trying to clean things up. But no luck so far. Black tank still shows full no matter what.

I suppose I could save myself the trouble and expense just by looking down the toilet bowl!!

i have the probes and have been wanting to make the modification. i also have the underbelly enclosed. went in from the top and found that i would have to drill holes (the old probes are bonded in). even with a close quarter drill, i didn't have enough room to get between the sewer out line and the tank.
couldn't see how the sewer out line was attach so it's on hold. i noticed from the pictures on herk's that it appears there is a hose clamp holding it. i may tackle it after all.
underbelly has a section that is removable below the tanks. i'll lower that next. it will be on my bucket list.
i've had success getting my black sensor to show zero after massive flushing. it is short lived though, first flush and all the way up.

where three of the wires attach to a small (postage stamp sized) box before the wire goes to the panel.

Jim, that "box" is a resistor pack.

See the attached diagram. As the indicators are submerged the resistor pack finds grounds and changes the resistance of the circuit. That is why a shorted signal wire will always show "FULL"

Are you sure that they are "bonded" in? There is a rubber compression washer on my old probes. A pair of side cutters were all that was required to wiggle them out. Be sure to push the old threaded part into the tank a little ways to provide room for the compressed rubber to collapse and allow withdrawal of the probe.